The hypothesis that a blood vessel's phenotype is determined by the target tissue being vascularized rather than by the source of the vessel is being tested further because it does not apply to mature tissue. When an autograft of mature skeletal muscle is vascularized by the fenestrated vessels of mature choroid plexus, the fenestrated phenotype is retained rather than being replaced by the muscle type. This exception may be due to the mature muscle graft being incapable of secreting a conversion factor or to the mature, ingrowing vessels being unresponsive to such a factor. To test this hypothesis, 13 ~ 14 day old fetal muscle, and choroid plexus still attached to the brain stem, are being cografted to the IV ventricle and cerebral or cerebellar cortex of adult rats. Fetal tissue is identified by its bromodeoxyuridine that had been injected into the pregnant rat. Although fetal muscle survives when grafted to the IV ventricle, very few of the grafted fetal choroid plexuses do. Cografts to the cerebral or cerebellar cortex are being attempted for better choroidal survivability and to differentiate donor from host choroid plexus without the need for a label. The mechanism of opening the blood~brain barrier (BBB) with RMP~7, a bradykinin analog, is also being studied. RMP~7 presumably opens the BBB only to small molecules such as sucrose and inulin (MW 5,000), by binding to a receptor on the luminal side of the endothelium. Attempts have begun to learn whether probe molecules such as biotinylated or fluorescein~conjugated dextran, MW 3,000, visible by light and electron microscopy, pass the barrier by receptor~mediated transcytosis initiated by the RMP~7.